302 DAKYNS : GLACIAL PHENOMENA OF WHARFEDALE. 
great mass of ice moved southward. The annexed figure represents 
what is seen : — 
a b represents the slope of the ground ; c d is a horizontal line. If 
this bending over was caused by ice it must have been by quite local 
ice ; for the general movement of the ice of the country was from 
b to a, not from a to B. As regards the drift material, it is of two 
sorts, well rounded and bedded sand and gravel, and ordinary boulder 
drift. This last is the most frequent ; it cannot always be called 
boulder clay, and in the higher part of the dale can seldom be so 
called except by courtesy, for it often consists of a mass of both 
rounded and angular stones, of all shapes and sizes, with very little 
clayey matter at all. The difference between it and boulder clay is, 
however, merely one of degree. When the drift is derived from hard 
rocks like grit and limestone, there will be a stony deposit with but 
little clay ; when it comes from shale, there will be much clay. 
It is quite impossible, as a rule, to separate the boulder beds 
from the sand and gravel. The boulder beds, when stony, approxi- 
mate to gravel in such cases as those in which the contained stones 
are well rounded ; and, moreover, as the section of the Barden Reser- 
voir shows, the sand and gravel is sometimes intercalated in the 
midst of boulder clay, consequently no general sequence can be made 
out ; but near Linton Mill the gravel certainly lies on the top of 
boulder clay, for there is a steep bank, the upper part consisting of 
dry gravel, and the lower of wet clay, which throws out the water 
percolating the first ; but as there is no section it is impossible to 
tell whether the gravel is a scratched boulder gravel or a water- 
washed gravel. Near Park Bridge, over the Wharfe, there is above 
the alluvial flat a bank of coarse gravel : though very rough and 
unstratified in places, it is on the whole distincely stratified. It 
